Digital-to-analog converters (referred to as DACs (Digital-to-Analog Converters) from now on) used for digital audio devices can be broadly divided into two types: 1-bit DACs used widely from low-end machines to quality machines; and multi-bit DACs employed by quality machines.
A multi-bit DAC prepares elements corresponding to bit weight of input digital data such as a PCM (Pulse Code Modulation) signal by the number of bits, and extracts the sum total as the output of the analog signal. In this case, the sampling frequency becomes a unit time of its operation.
In contrast, a 1-bit DAC has only two types of output voltage (0 or 1 output) and employs a system of expressing the amplitude along a time axis by increasing the unit time of its operation to several hundred times of the sampling frequency. As a sampling system employed by a 1-bit DAC, a ΔΣ (delta-sigma) system has been known. The ΔΣ (delta-sigma) system is a name of a sampling system that carries out higher operation in the time axis direction to perform processing corresponding to multi-bit operation by the 1-bit DAC.
Being different from the multi-bit DAC, since the 1-bit DAC does not use weighting elements, it has higher linearity in micro amplitude. However, since it drives the quantization noise beyond the audio band (20 KHz) by a noise shaping technique, noise in a high-frequency band increases comparatively. The 1-bit DAC usually performs DA conversion of the total of about 16-18 bits by combining with 2 to 4 order noise shaper.
As an example of the noise shaping technique described above, there are a MASH (registered trademark) system and some others, and as an example of the 1-bit DAC system, there are a PWM (Pulse Width Modulation) conversion and some other methods.
The PWM conversion is a technique that varies its output voltage by the pulse width, which outputs a pulse signal with the width corresponding to the values of the input signal. Here, the PWM output waveform includes amplitude components determined according to its pulse width and phase components determined according to its power center. Thus, if the PWM conversion is made in disregard of the phase components, a PWM output waveform whose sampling intervals are random is output, and the output waveform is distorted inevitably.
For this reason, 1-bit DAC PWM circuits have been conventionally proposed which performs control in such a manner that the waveforms of the power centers of the individual PWM output waveforms become identical in the individual intervals to prevent occurrence of distortion (see Non-Patent Document 1 and Patent Document 1, for example).